In electrochemical elements with lithium-intercollating electrodes in the form of rechargeable 4 V cells, the separator carries out a large number of extreme tasks. The negative electrode, which is normally based on carbon, has a lithium activity close to unity when the cell is in the fully charged state, which is generally 4.2 V. This means that, although the active material is a carbon modification, the reduced potential with the fully charged cell can be compared with that of metallic lithium. On the positive side, lithium transition metal oxides are generally used as the active material. When the cell is fully charged, this results in a very strong oxidation potential, which only a small number of substances withstand. Furthermore, the stability of a fully charged cell must not be adversely affected in a damaging manner even at increased temperatures, at least in the short term, in which context it is necessary to consider temperatures of up to 90° C. and a time period of at least two or more hours.
For this reason, polyolefin separators which are formed, for example, from polypropylene (PP), polyethylene (PE) or from two or more layers of these materials, are standard in rechargeable lithium cells. With uniaxial or biaxial stretching, these are provided with the necessary pore structure for holding the liquid electrolyte and, by way of example, the melting point of PE at about 120° C. is made use of to melt these pores in the event of an inadvertent short circuit or a rise in the temperature in the cell interior caused in any other way, thus resulting in a so-called “shut-down effect.” This is a drastic rise in the internal resistance, since the closing of the pores means that the liquid electrolyte no longer provides conductivity in the separator.
EP 0 951 080 B1 describes a battery separator comprising a first and a third microporous layer, with a second microporous membrane with a shut-down capability between them, and with the porous membranes being produced using a stretching process.
However, it has been found in practice that this effect does not always work reliably. In the event of a very short and severe temperature rise, the shut-down mechanism may no longer work in some circumstances. Drastic temperature rises such as these may occur in particular when the cell is poked from the outside with a conductive sharp object, and in the event of overcharging of the cell. This negative reversal of the shut-down mechanism is associated with the polyolefin separators shrinking during melting. The greater and stronger the sudden energy rise, the greater is the shrinkage process. This results not only in pores being closed, but in the entire separator collapsing by shrinkage in a disadvantageous manner such that the cell is short-circuited and, thus, reacts even more strongly, with possible ignition of the cell. The method of producing uniaxial or biaxial stretching to produce pores in the separators may in this case also have a disadvantageous effect since the separator collapses in the form of shrinkage like a spring in the stretching direction as a result of the memory effect, which is associated with the production process. Particularly in the case of large lithium cells, such as those for 42 Volt vehicle power supply systems or traction purposes, a solution to the separator problem as described above is essential.
EP 1 096 591 A1 describes a gel-like separator membrane with a binder polymer from the group comprising polyacrylonitrile, polymethyl methacrylate, polyvinyl chloride, polyvinyl sulphone, polyethylene glycol diacrylate, polyvinylpyrrolidone, polyvinylidene difluoride or mixtures thereof with a ceramic material chosen from Al2O3, SiO2, TiO2, ZrO2, or mixtures thereof.
EP 1 043 795 A2 describes a composite electrode, which is formed from active electrode material, a conductivity enhancer, a polymer electrolyte and a mineral, naturally occurring solid electrolyte.
U.S. Pat. No. 6,057,061 discloses a battery separator comprising an oriented microporous film which contains ethylene vinyl alcohol (EVA) and has a chemically inert filling material chosen from glass-like ceramic, polytetrafluoroethylene (PTFE) or condensed plastic particles having rubber-like characteristics.
EP 1 011 157 A2 describes a separator material for rechargeable lithium batteries using a polyvinylidene difluoride hexafluoropropylene as the binding agent, in which particles with a higher softening temperature are dispersed.
DE 199 16 109 A1 describes composite bodies which are suitable for use as separators in electrochemical cells, with at least one first layer which contains at least one solid being applied to at least one second layer composed of a conventional separator material.
DE 200 10 080 U1 describes a rechargeable battery based on solid ion conductors with a solid electrolyte arranged between the positive and negative electrodes, with the negative electrode also being formed from carbon nanostructures.
It would therefore be advantageous to provide a method for producing an electrochemical element with fire protection characteristics, in which dangerous shrinkage of the separator in response to a short-term very severe temperature rise in the cell is prevented and in which the cell is prevented from igniting, but whose separator at the same time has better ionic conductivity.